Biofiltration process and apparatus for odour or voc treatment

ABSTRACT

A bioreactor and process is described which offers a transition or continuum between one or more of (a) from biotrickling filter conditions to biofilter conditions, (b) changing media characteristics, c) liquid or nutrient recirculation rates or frequency, (d) pH, (e) gas velocity, (f) retention time along the gas flow passage or (g) cross-sectional area. For example, media may be arranged in sections of a rectangle, with gas flow in a horizontal direction sequentially through the sections, and liquid flow in a vertical direction from top to bottom in one or more sections. Zonal control of process conditions may be provided as the gas passes from inlet to outlet and liquid is introduced at the top of one or more zones and flows down by gravity.

This application claims the benefit of U.S. Provisional PatentApplications 60/977,493 (filed on Oct. 4, 2007), 60/979,605 (filed onOct. 12, 2007), and 60/979,619 (filed on Oct. 12, 2007), each of whichare incorporated herein in their entirety by this reference to them.

FIELD

This specification relates to the treatment of gases using biotricklingand biofiltration.

BACKGROUND

The following is not an admission that anything described below iscitable as prior art or part of the common knowledge of persons skilledin the art.

Odours generated from diverse sources such as wastewater treatmentplants, pet food processing, biosolids and municipal solid wastecomposting, rendering of animal fat, etc are a nuisance for thesurrounding population, and plant operators must provide treatment.Technologies for treatment of these odours include bioscrubbers,biofilters, biotrickling filters, carbon adsorbers, and chemicalscrubbers. Each of these solutions has its own advantages anddisadvantages, and all of these result in significant capital andoperating costs with no recovery of valuables. Sometimes the impuritiesare present at such a high level, or the exhaust concentrations limitsare so low, that a number of these operations must be installed inseries. In some cases, different impurities, that might inhibitbiodegradation of each other, or require different process conditions,may force the use of the same unit process twice to permit operationunder optimum conditions for each impurity to obtain acceptable removal.

Biofiltration has been used to treat odours. In an example,biofiltration involves a vessel with an organic or inorganic media,through which odorous gases are passed. The media is typicallybiologically active and inorganic media may be coated to make itbiologically active. A biofilm grows on the surface of the media, andodour causing compounds such as hydrogen sulphide are oxidized intoodourless or low odour compounds. As many of the odour causing compoundsare difficult to biodegrade, long residence times of up to 1 minute areoften required. This results in very large equipment, large spacerequirements and high costs. Organic media, such as compost which istypically used in biofilters, is easily degraded, resulting inchannelling, high pressure drop, and poor treatment and must be replacedperiodically.

Another method of treating odorous gases, for example hydrogen sulphide,is a biotrickling filter. This is analogous to the biotrickling filterused for liquid wastewater treatment, except the air contains thecontaminants instead of water. These systems, while more compact thanthe biofiltration example described above, require gas collection andliquid recirculation. Since high treatment rates are the key objective,conditions are optimized for a selective contaminant, and may not besuitable for others. For example, for off-gases containing highconcentration of hydrogen sulphide and low concentrations of otherreduced sulphur compounds such as dimethyl sulphide, methyl mercaptan,dimethyl disulphide, and carbon disulphide, the filter may be operatedat a pH in the range of 1.5 to 2 to obtain high treatment rates forhydrogen sulphide. But biological treatment of other reduced sulphurcompounds is poor under these conditions and inadequate treatment isachieved. The process is particularly ineffective for biosolids systemexhausts where a combination of hydrogen sulphide, reduced sulphurcompounds, reduced nitrogen compounds and other volatile hydrocarbonsmay be present.

U.S. Pat. No. 6,790,653, U.S. Pat. No. 5,891,711, US patent Publication2003/0027325, Gholamreza Moussavi et al (Journal of Hazardous Materials,2006), Madjid Mohseni et al, (Journal of Chemical Technology andBiotechnology, 2005), U.S. Pat. No. 6,632,659, and, Huiqi Duan et al(Applied Microbiology Biotechnology, (2005), 67: 143-149) also describetreatment processes.

SUMMARY

The following summary is intended to introduce the reader to the moredetailed discussion to follow. The summary is not intended to define orlimit the claims.

A system and reactor are described that combine multiple steps or zones,optionally in an integrated system or reactor. The zones may varyrelative to each other, or there may be variations within a zone, orboth, in relation to one or more characteristics. The varyingcharacteristic may be one or more of cross sectional area perpendicularto flow direction, media, or process type or parameters. For example,the reactor may have multiple process zones including two or more of abiotrickling zone, an intermittent biotrickling (transition) zone, and abiofilter zone in series in a horizontal flow configuration.Alternately, the reactor may have multiple process zones including twoor more of a biotrickling zone, an intermediate biofilter zone, and abiofilter zone in series in a horizontal flow configuration.

A reactor and process are described which offer a transition orcontinuum between one or more of (a) a transition between two or more ofbiotrickling filter (BTF) conditions, intermittent biotrickling filterconditions, and biofilter (BF) conditions, (b) a transition between twoor more of biotrickling filter conditions, intermediate biofilterconditions and biofilter conditions, (c) changing media characteristics,(d) liquid or nutrient solution recirculation rates or frequency, (e)pH, (f) gas velocity, (g) retention time along the gas flow passage or(h) cross-sectional area. For example, the media may be arranged insections of a tank, with gas flow in a horizontal direction through thetank, and liquid flow in a vertical direction from top to bottom. Zonalcontrol of process conditions may be provided as the gas passes frominlet to outlet and liquid is introduced at the top of one or more zonesand flows down by gravity.

Optionally, one or more physical/chemical treatment processes arelocated either upstream of the biological process or downstream. Thismay further improve the range of contaminants that can be treated. Thecomplementary processes may include ultraviolet treatment, non-thermalplasma, ozone or water, chemical or biological scrubbing on the inletside of the biofiltration system or activated carbon or anotheradsorption process on the outlet side of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic see-through isometric drawing of an example of areactor, with a top cover off;

FIG. 2 is a schematic drawing of an underside of an example of top coversuitable for use with the reactor of FIG. 1; and

FIG. 3 is a partial schematic see-through isometric drawing of anotherexample of a reactor.

DETAILED DESCRIPTION

Various apparatuses or processes will be described below, including anexample of each claimed invention. No example described below limits anyclaimed invention and any claimed invention may cover processes orapparatuses that are not described below. The claimed inventions are notlimited to apparatuses or processes having all of the features of anyone apparatus or process described below or to features common tomultiple or all of the apparatuses or processes described below. It ispossible that an apparatus or process described below is not an exampleof any claimed invention. Rights to file continuing applications arereserved in any invention disclosed in an apparatus or process that isnot claimed in this document. Any one or more features of any one ormore examples can be combined with any one or more features of any oneor more other examples.

An integrated process, system, and reactor with gradual or othertransitions may treat exhaust gases containing volatile impurities orone or more gases that may cause unpleasant odours or may be hazardousin nature. Examples of gases that might be treated include but are notlimited to reduced sulphur compounds such as hydrogen sulphide, methylmercaptan, carbon disulphide, dimethyl sulphide and dimethyl disulphide;volatile organic carbons such as aromatic hydrocarbons, esters,aliphatic hydrocarbons and chlorinated hydrocarbons; ammonium compoundssuch as ammonia, trimethyl amine, indole and skatole; hazardous airpollutants such as methanol and formaldehyde and other volatilecompounds. There may be synergistic treatment of different contaminantsin a single vessel using more than one unit process.

Gas flow in a horizontal direction in a reactor, with multiple zoneswith, for example, different process conditions and/or types of mediamay help to do one or more of the following: increase the ability of thesystem to handle gases with dust and other impurities, may avoid use ofhumidifier, provide greater ability to manage biofilm, enable operationwith minimum drainage for retention of microorganisms for treatingminor, yet highly odorous contaminants such as dimethyl sulphide, andmay minimize footprint and gas side pressure drop. Liquid flow from topto bottom perpendicular to the gas flow may permit establishment ofdifferent process zones along the gas path. Progressively finer mediamay be used from inlet side to the outlet side to provide roughingtreatment at the inlet where concentrations are high, and high level oftreatment on the outlet side where concentrations are very low. Forexample, a coarse media may be used in the inlet section wherebiotrickling filter conditions predominate, and a fine media may be usedon the outside for the biofilter. Optionally or additionally, an inertmedia may be used closer to the inlet for removal of high concentrationsof specific contaminants, and a biologically active media may be used onthe outlet side for enhanced removal of trace contaminants. In exampleswherein an intermediate biofilter zone and a biofilter zone areprovided, the media used may be different between the intermediatebiofilter zone and the biofilter zone. For example, a coarse,biologically active media may be used in the intermediate biofilter zonewhere the highest contaminant concentrations exist and a finerbiologically active media may be used in the biofilter zone where verycontaminant concentrations may exist and plugging by biofilm may not bea concern. When high levels of VOCs are present, media with high voidvolume and surface may be used, and may be installed in a manner tofacilitate manual cleaning of the excess biofilm. When dictated by thetypes of contaminants present, the biofilter section may be furtherdivided into zone with different types and sizes of biologically activemedia.

In some examples, the cross section of each zone is constant, forexample in a rectangular reactor. In other examples, the cross sectionvaries between each zone, for example in a cylindrical reactor withconcentric zones.

A process or system may provide a gradual transition from a biotricklingfiltration process or zone to a biofiltration process or zone. This mayhelp deal with a range of impurities that cannot be treated aseffectively by a single process or zone, and to increase the efficacy ofuse of reactor volume.

A process or reactor may provide liquid flow from top to bottom in oneor more zones. There may be a gradual transition in the nature, flowrate and/or frequency of trickle flow. For example, continuous tricklerate with recycle may be used on the inlet side and infrequent or no,irrigation may be used on the outlet side. Also, the trickle liquid maybe collected and recirculated on the inlet side to control pH, and freshwater may be used infrequently on the outside. Within the differentbiofilter zones, different rates and frequencies of irrigation may beused. Typically, a biotrickling zone is in communication with the inlet,for example adjacent and horizontally spaced from the inlet, to removebulk of the high concentration impurities such as hydrogen sulphidepresent in sewage plant exhausts at an optimum pH of 1.5-2.0. This maybe followed by an intermittent biotrickling zone in communication withthe biotrickling zone, for example adjacent and horizontally spaced fromthe biotrickling zone where trickle rate may be lower or intermittent toprovide the ability to remove the balance of hydrogen sulphide andammonia, while effectively flushing out the sulphuric acid and nitricacid by-products, and maintaining a near neutral pH to encourage growthof heterotrophic species for the removal of contaminants such as methylmercaptan, dimethyl sulphide, and volatile organic compounds includingmethanol, dibutyl acetate, xylene, toluene, methyl mercaptan and others.A biofilter zone may be provided in communication with the intermittentbiotrickling zone, for example adjacent and horizontally spaced form thebiofilter zone, where a very small amount of irrigation may be used topreserve bacteria able to degrade dimethyl sulphide, one of the moreodorous compounds, which is typically present in very smallconcentrations, and high irrigation rates may prevent its treatment bywashing off nascent microorganisms. Alternately, rather than anintermittent biotrickling zone, an intermediate biofilter zone may beprovided in communication with the biotrickling zone and the biofilterzone. The intermediate biofilter zone may have a higher irrigation rateto flush out byproducts, and the biofilter zone may have very low or noirrigation to preserve biomass to help provide very high level oftreatment. Similar strategy is used to enhance treatment of difficult todegrade VOCs such as alfa pinene, commonly present in exhausts forplywood and oriented strand board operations.

A pre-treatment step such as aqueous or chemical scrubbing may be usedupstream of the biological processes for contaminants that are not watersoluble or not easy to degrade using biological process, or for removalof particulate matter, or for removal of condensable impurities such asorganic compounds present in many hot gas streams.

Photo-oxidation using ultraviolet light may be provided upstream of thebiological processes for contaminants that are hydrophobic or difficultto biodegrade, to convert these to hydrophilic, or more easilybiodegradable molecules.

Gases may be pre-treated using ozone gas upstream for contaminants thatare highly hydrophobic or not easy to remove using biological process.This may help break these down into simpler to biodegrade compounds.

Gases may be pre-treated using non-thermal plasma upstream forcontaminants that are highly hydrophobic or not easy to remove usingbiological process. This may help break these down into simpler tobiodegrade compounds.

A post-treatment polishing step may be used such as activated carbon inthe same vessel or in a separate vessel as the biological processes.This may help achieve a high level of treatment.

Alternately, gases may be post-treated using non thermal plasma.

Trickle water may be recycled to an inlet side biotrickling section tomaintain a relatively uniform low pH condition for rapid hydrogensulphide oxidation. Once through liquid flow may be used to maintainnear neutral conditions in the intermittent biotrickling zone and thebiofiltration zones, for example to oxidize reduced sulphur compounds.

Once through trickle liquid may be used when highly soluble gases suchas ammonia are present in the inlet gas stream to use it as acombination of biotrickling filter and scrubber.

A fine water mist may be sprayed in the inlet plenum when a solublecontaminant such as ammonia is present in the inlet gas. The mist isthen removed on the media in a biotrickling zone and is discharged withthe once through trickle liquid.

A venturi scrubber may be installed upstream of the biotrickling zone toremove particulate matter and condensable matter.

A perforated inside air plenum may be provided to feed air to the mediabed, and perforated outside tank wall to discharge treated air, and asolid floor and tank ceiling to ensure that the gas flow is horizontal.

A reactor may be operated under negative pressure to provide fullcontainment of the gases.

A reactor may be operated under positive pressure to simplify exhaustair discharge and to reduce system cost by eliminating the outletplenum.

A method and a system for the treatment of a gas, for example a gas thatcontains odour causing chemicals as present in exhaust gases fromdifferent types of operations, may consist of one or more of thefollowing steps:

-   -   1. Receiving odorous or toxic exhausts, either under suction or        positive pressure.    -   2. Providing a generally horizontal air flow/generally vertical        liquid flow odour treatment system, with conditions similar to        biotrickling filter (i.e. a biotrickling zone) upstream and        conditions similar to biofilter (i.e. a biofilter zone)        downstream, and a transition intermittent biotrickling zone in        between. This transition is achieved by selecting an inert media        for the biotrickling zone and the intermittent biotrickling        zone, and biologically active media for the biofilter zone, by        recirculating the liquid to the upstream section continuously        and providing once through, intermittent irrigation on the        downstream end, and at a gradually decreasing flow and frequency        in the intermittent zone. Alternately, the treatment system may        include a biotrickling zone upstream, and biofilter zone        downstream, and an intermediate biofilter zone between the        biotrickling zone and the biofilter zone. The biotrickling zone        may have an inert media, and the intermediate biofilter zone and        the biofilter zone may have a biologically active media. The        intermediate biofilter zone and the biofilter zone may differ in        irrigation rates, and may include different media. For example,        the intermediate biofilter zone may contain coarser biologically        active media, and a high irrigation rate, and the biofilter zone        may have a finer media and a low irrigation rate. This may avoid        biofilm formation and plugging in the intermediate biofilter        zone while flushing out byproducts, whereas plugging may not be        a concern in the biofilter zone. For example, the intermediate        biofilter zone may be irrigated at a rate of 0.01-0.1 ft3 of        water/ft3 of media per day, total, delivered in equal        proportions for 1 to 24 times per day, and the biofilter zone        may be irrigated at a rate of 0.001-0.01 ft3 of water/ft3 of        media per day, total, delivered in equal proportions for 0 to 3        times per day. More specifically, the intermediate biofilter        zone may be irrigated at a rate of 0.03-0.07 ft3 of water/ft3 of        media per day, total, delivered in equal proportions for 1 to 24        times per day, and the biofilter zone may be irrigated at a rate        of 0.003-0.007 ft3 of water/ft3 of media per day, total,        delivered in equal proportions for 0 to 3 times per day.    -   3. Introducing the gases into the central gas distribution        manifold of the concentric treatment device.    -   4. Transferring gas from the central manifold to the        biotreatment system in a horizontal direction, where treatment        of hydrogen sulphide, reduced sulphur compounds, volatile        organic carbon compounds, and other gases takes place at a high        rate.    -   5. Transferring gases from the biotrickling zone to the        intermittent biotrickling zone to the biofilter zone in a        horizontal direction, where further treatment of residual        contaminants takes place. Alternately, transferring gases from        the biotricking zone to the intermediate biofilter zone to the        biofilter zone in a horizontal direction, where further        treatment of residual contaminants takes place.    -   6. Discharging gases to the outside from perforated biofilter        wall, or from a manifold placed outside of the biofilter to        collect the gas prior to discharge through a stack.    -   7. Introducing liquid at the top of the biotrickling zone        continuously, and distributing it evenly across the gas flow        path, but differentially along the gas flow path, using spray        nozzles or perforated pipes    -   8. Introducing liquid at the top of the intermittent        biotrickling zone intermittently, or the intermediate biofilter        zone intermittently, and distributing it evenly across the gas        flow path, but differentially along the gas flow path, using        spray nozzles or perforated pipes. The frequency of liquid        recirculation may depend on many factors, including level of        contaminants, pH of the drainage solution, etc.    -   9. Introducing irrigation liquid at the top of the biofilter        zone periodically. This may also depend on levels of        contaminants. In some cases, where very high level of treatment        of trace components is desired, very infrequent irrigation may        be provided.    -   10. Collecting drainage from biofilter and biotrickling zones in        a sump and recycling the drainage to the top of the biotrickling        zone using a recirculating pump. Excess water may be discharged        from the sump to maintain desirable pH conditions.

FIGS. 1 and 3 show alternate examples of a reactor 100. The reactors maybe designed to treat gases from, for example, a municipal waste waterplant having up to 50 ppmv of H₂S, although it may be used or modifiedfor other applications. Each reactor comprises a biotrickling zone 102,an intermittent biotrickling zone 104, and a biofilter zone 106.However, in alternate examples, only a biotrickling zone and a biofilterzone may be provided. Further, in alternate examples, a biotricklingzone, an intermediate biofilter zone, and a biofilter zone may beprovided. In such examples, the reactor would be similar to reactor 100,however the intermediate biofilter zone would include biologicallyactive media, rather than inert media. Further, in alternate examples, abiotrickling zone, an intermittent biotrickling zone, an intermediatebiofilter zone, and a biofilter zone may be provided. The term“biotrickling zone” refers to a zone operated like a biotricklingfilter. The term “intermittent biotrickling zone” refers to biotricklingfiltration zone with intermittent, but frequent irrigation to removecontaminants such as hydrogen sulphide and ammonia, that produce acidicby products. The term “biofilter zone” refers to a zone which operateslike a biofilter, with infrequent irrigation at a low rate to preservebiomass and to reduce losses of heterotrophic bacteria.

Referring to FIG. 1, in the example shown, the reactor 100 has a centralgas distribution manifold or plenum 108 with two identical biofiltrationsystems 110 a, 110 b, on opposed sides of the central plenum 108. Thefirst system 110 a is adjacent and horizontally spaced from a first side120 a of the plenum 108, and the second system is adjacent andhorizontally spaced from the second side 110 b of the plenum 108. Inalternate examples, the reactor may only comprise one biofiltrationsystem.

Contaminated air is supplied to the central plenum 108, which may be,for example, sized for a gas velocity in the range of 500 to 2500ft/minute. A perforated wall 112 permits the gas to pass to the nextzone of the reactor 100, which is a biotrickling zone 102. Thebiotrickling zone 102 is in communication with and horizontally spacedfrom the plenum 108. A coarse, inert packing material may be in zone102. For example, the packing material may include a lightweight mediamade of expanded glass, for example, PORAVER™ granules as made byPoraver North America, Barrie, Ontario, Canada. To make a biologicallyactive media, the granules may be coated. Various media which may beused are described in U.S. patent application Ser. No. 10/687,761 filedOct. 20, 2003, U.S. patent application Ser. No. 11/583,783 filed Oct.20, 2006, U.S. patent application Ser. No. 11/542,107 filed Oct. 4, 2006and U.S. patent application Ser. No. 12/245,327 filed on Oct. 3, 2008,all of which are incorporated herein in their entirety by this referenceto them. Such lightweight media is generally non-compacting, less densethan many other types of non-compacting media, and so allows a tallreactor to be constructed, for example 4 meters high or more.

Velocity in zone 102 may range from 5 ft/minute 30 ft/minute. Totalempty bed residence time (EBRT) may range from 2 seconds to 30 seconds.

Trickle liquid is introduced via line 212 from the top 214 of zone 102using perforated pipes or spray nozzles 216, which may be provided on atop cover 218 of the reactor 100 (shown in FIG. 2), and the air flowsacross the liquid in a generally horizontal direction, indicated byarrows A. Further, baffles 218 may be provided to separate the zones.Treatment of high concentration of contaminants such as hydrogensulphide takes place in zone 102 at a high rate. However, as the gastravels along the length of the reactor, in the direction indicated byarrows A, the pH conditions are changed by adjusting the trickle liquidcharacteristics, rates, and frequency.

The intermittent biotrickling zone 104 is horizontally spaced from andin communication with the biotrickling zone 102. In the intermittentbiotrickling zone 104, conversion of residual hydrogen sulphide tosulphuric acid and water is generally progressed and ammonia to nitricacid and water are nearly completed. Significant conversion of reducedsulphur compounds and other volatile organic matter also starts in zone104. The intermittent biotrickling zone may also include inert media.

In communication with and horizontally spaced from the intermittentbiotrickling zone 104 is the biofilter zone 106. In the biofilter zone106, which preferably contains a biologically active media, the residualreduced sulphur compounds and volatile organic compounds are treated.This media may be of a finer size than the biotrickling filter media toprovide high surface area intensity, and may contain additives such asactivated carbon, pH buffering agents, nutrients and compost to supportbiological growth. Biotrickling filter media may be generally 8-16 mm,as sized by screens with openings of those sizes, whereas biofiltermedia may be generally 4-8 mm size.

The reactor 100 shown in FIG. 1 may result in a substantial reduction inempty bed retention time as compared to both biotrickling filter and abiofilter, where a combination of these processes is used for treatmentof gases containing hydrogen sulphide and reduced sulphur compounds. Asused herein, empty bed retention time (EBRT) refers to the contact timebetween the gases and the biofilter media. For example, in an off gasstream from a sewage pumping station, the biotrickling zone 102 isdesigned for an empty bed retention time of 1-10 seconds, depending onthe nature and concentration of hydrogen sulphide. Similarly, theintermittent biotrickling zone 104 is designed for a retention time of1-10 seconds, depending on the level of hydrogen sulphide and ammonia.Biofilter zone 106 is then designed for an empty bed retention time of5-30 seconds to treat reduced sulphur compounds. In this invention,because of the overlap between the biotrickling zone 102 and thebiofilter zone 106, and because of gradual change in process conditions,parallel treatment can occur and it is estimated that the biotricklingzone 102 EBRT can be reduced by up to 30%.

In some examples, the inlet plenum 108 can be retrofitted with apre-treatment step, depending on the nature of contaminants present. Forexample, for a water-soluble impurity such as ammonia, which is noteasily degraded through bioreaction, the inlet plenum 108 may be packedwith media with very high void space and water may be used to scrub outthe impurity. In another example, ultraviolet treatment may be providedfor difficult to biodegrade compounds such as chlorinated hydrocarbonsand aromatics. In such cases, the ultraviolet system may be located inthe inlet plenum 108 or outside in a separate treatment system. Inanother example, ozone may be used to oxidize impurities such aschlorinated hydrocarbons, which are difficult to biodegrade, and can beoxidized to easily biodegrade molecules. Ozone would likely be added ina separate reactor because of safety and materials considerations. Inanother example, non thermal plasma may be used to pretreat the gases.

The generally horizontal configuration of FIG. 1, wherein gas passesgenerally horizontally through each of the zones, may have manybenefits:

1. It may be possible to use only 3-5 ft media bed depth along the gasflow path without increasing foot print by increasing the height of thebed. The pressure drop can be reduced, providing buffer for pressuredrop build up due to sulphur accumulation in the biofilter zone underhigh H₂S concentration episodes.

2. The biotrickling filter zone 102, using 8-16 mm LWE media, may becompletely integrated with the biofilter zone 106 at little additionalcost.

3. Flexibility is available to use different irrigation strategies alongthe air flow path. This may improve performance for methyl mercaptan,DMS, amines and other highly odorous but low concentration compounds.Tools for this include use of very low irrigation rates in the biofilterzone 106 for biomass preservation, and pH management.

Integration of the biotrickling zone with biofilter zone may make itpossible to:

1. Use even lower biotrickling filter and biofilter zone EBRTs forapplications with high H₂S feed even if high total odour reduction isrequired;

2. Reduce or eliminate humidification as a biotrickling filter, alongwith recirculation solution temperature management, can be used for +95%humidification. Heating may further improve the biotrickling zone H₂Stolerance by increasing reaction rates. Note that an evaporativepre-humidifier may be required for tropical climates;

3. Feeds may be treated with up to 200 pppmv (or even higher—ifrecirculation temperature management is feasible) H₂S using 8-16 LWE;

4. The biofilter zone may be protected from sulphur deposition;

5. The need for biofilter irrigation may be reduced, minimized, oreliminated; and,

6. If present, ammonia may be adsorbed in the biotrickling zone usinghigh trickle and purge rates.

In some examples, media settling may be reduced by selecting media thatis highly uniform, for example with a uniformity coefficient of about 2or less. Uniformity coefficient is the ratio of the size of particlethat has 60 percent of the material by weight finer than itself, to thesize of the particle that has 10 percent (by weight) finer than itself.This restricts movement of particles and so reduces settling ofparticles which might otherwise result in non-uniform air flow andtreatment. Loose carbon fines may be washed off initially to the bottomand may drain away. The low media density may permit pre-filling thetank at the fabricator prior to shipping, resulting in reduceddegradation during handling. Also, to minimize settling, anon-compressible media may be used. Examples of such media include lavarock, expanded glass beads, or structured media such as HD QPac,manufactured by Lantic Packing. A biotrickling filter upstream of thebiofilter may humidify air completely, and may reduce the importance ofirrigation and uniform moisture distribution across the air flow path.

To ensure that the biotrickling filter zone humidifies air to a greaterthan 90% relative humidity, a sump, with a pump 220 and a heater may beprovided (shown in FIG. 2). The degree of heating is expected to beminimal compared with heating required during winter to preventfreezing. Trickle liquid may be introduced through either buried porouspipes or through nozzles. There may be hydraulic separation between thebiotrickling zone and biofilter zone to minimize leakage of biotricklingsolution into the biofilter zone, for example using baffles 218.Similarly, the biofilter zone may be divided into two sections (i.e. anintermediate biofilter zone and a biofilter zone) permit use ofdifferent media, or different irrigation schedules. For tropicalclimates, evaporative cooling/humidification may be more energyefficient, and a separate humidifier may be provided.

Sample dimensions and other parameters are provided in Table 3.

TABLE 3 Design of Sample Horizontal Biofilter Inlet gas hydrogensulphide concentration 50 ppm average, peaking to 100 ppm Inlet gasorganic sulphur compounds 0.5-10 ppm Inlet gas odour 10,000 to 100,000D/T Hydrogen sulfide removal  99% Organic sulphur compounds removal >90%Total odour removal >90% Tank height, ft 6 Tank width, ft 5 Tank length,ft 6 BTF width, ft 1 Air Flow, cfm 440 BTF Design Media Expanded glassbeads, 8-16 mm diameter or structured media such as HD QPac by LanticProducts BTF X-sectional area, ft2 27.5 BTF length, ft 1 Velocity, fpm16 BTF volume, ft3 27.5 BTF Residence time, sec 3.75 BTF media PD, inWater column 1.5 BTF trickle rate 0.7 to 1.4 gpm/ft2 of reactor surfaceBTF trickle liquid pH 1.5-2 BF Zone 1 (BF 1) Design Media Biosorbens XLDBF 1 height, ft 5.5 BF 1 Width, ft 5 BF1 Cross-sectional area, ft2 27.5Velocity, fpm 16 Pressure drop, in WC/ft 0.23 EBRT, s 7.5 BF 1 mediaVolume, ft3 55 BF 1 Length, ft 2 Irrigation rate, ft3 of water/ft2 ofmedia surface/day 0.05 Irrigation application frequency 3 times/day BFZone 2 (BF 2) Design Media Biosorbens XLD BF 2 height, ft 5.5 BF 2Width, ft 5 BF2 Cross-sectional area, ft2 27.5 Velocity, fpm 16 Pressuredrop, in WC/ft 0.23 EBRT, s 75 BF 2 media Volume, ft3 55 BF 2 Length, ft2 Irrigation rate, ft3 of water/ft2 of media surface/day 0.005Irrigation application frequency 1 times/day

An alternate example of a reactor 300 is shown in FIG. 3. The reactor300 operates in a similar fashion to reactor 100, however, only abiotrickling zone 302 and a biofilter zone 306 are provided. Further,the reactor 300 defines a generally vertical cylinder, and the zones areconcentric, with outside-in gas flow. Dividers, for example plastic meshdividers, may be provided between the zones. The reactor 300 includes anouter concentric zone defining a gas inlet plenum 308. A perforatedcylindrical wall 312 permits the gas to pass inwardly from the inletplenum 308 into biotrickling zone 302, which is in communication withand horizontally spaced from the inlet plenum 308. The biotrickling zonehas a low pH and is provided with coarse LWE media for elemental sulphurcontrol. Drainage is collected from the biotrickling zone 302 in a sump320 and is recycled to the top of the biotrickling zone via line 322using a recirculating pump 324. Excess water may be discharged from thesump via line 326 to maintain desirable pH conditions (e.g. between 1.5and 2). From the biotrickling zone 302, the gas passes inwardly to the abiofilter zone 306, which is in communication with and horizontallyspaced from the biotrickling zone 302. The biofilter zone may have aneutral pH and fine LWE media for high removal of total reduced sulphurcompounds. The biofilter zone 306 may be provided with once-throughsecondary effluent or tap water, via line 328, for neutral pHconditions. If the water is re-circulated, the pH may be controlled tonear neutral conditions. From the biofilter zone 306, the treated gaspasses into a central outlet manifold 318.

EXAMPLE 1

A biofiltration system was tested with a 4-8 mm expanded glass coatedmedia for treatment of hydrogen sulphide bearing air. A 3-ft deep bed inan 8 inch diameter column was tested to simulate the conditions in a lowbed depth biofilter. Conditions were as a follows:

Apparent air velocity: 11.5 ft/min

Bed depth: 3 ft

Inlet H₂S concentration: 31.2 ppmv

Temperature: 81 F

Outlet and mid point concentrations were measured. Results are asfollows:

Mid point, representing 7.5 second empty bed residence time: 4.7 ppm v

Outlet, representing 15 second empty bed residence time: 0 ppm v

This demonstrates that at low air velocity and shallow bed depth,similar to that proposed for the concentric design, very high removalefficiencies are achievable in a biofilter.

EXAMPLE 2

A biotrickling filtration system was tested with an 8-12 mm expandedglass uncoated media for hydrogen sulphide treatment. The objective wasto test velocities that would typically exist in the reactor with BTF.Operating conditions and results are as follows:

Apparent air velocity: 23 ft/min

Bed depth: 3 ft

Inlet H2S concentration: 99-100.9 ppm v

Temperature: 74.3

Outlet and mid point concentrations were measured. Results are asfollows:

Mid point, representing 4 s EBRT: 9.3 ppm v

Top point, representing 8 s EBRT: 4.2 ppm v

This indicates very high performance at typical EBRT and velocity in theBTF section of the reactor. It would appear that a 4 s EBRT in BTF,followed by 15 s EBRT in BF will provide very high removal of H2S andother reduce sulphur compounds.

EXAMPLE 3

The biofiltration system of Example 1, using process conditions ofExample 1, but with no hydrogen sulphide and 5 ppm of dimethyl sulphidewas tested after about three weeks of acclimation. DMS was added for an8-hour period. DMS is considered to be one of the most difficultcompounds to remove. 51% removal was achieved.

A system or process as described above may result is a high level oftreatment in an efficient manner, such as for gases containing a mixtureof hydrogen sulphide and reduced sulphur compounds, and where acidgenerated by hydrogen sulphide treatment might otherwise preventbiological degradation of reduced sulphur compounds such as methylmercaptan and dimethyl sulphide. System pressure drop may be low, whileproviding high velocity at the points of high reaction rates.

EXAMPLE 4

A horizontal system with a biotrickling zone, an intermediate biofilterzone and a biofilter zone, all arranged in series, with air passing frominlet plenum to the biotrickling zone to the intermediate biofilter zoneand the biofilter zone, and finally to an outlet plenum was tested.Dimensions of the horizontal flow system are as follows:

Operating Conditions:

-   -   Overall media Bed Dimensions: 5.5 ft [1.7 m] high×5 ft [1.7 m]        wide by 5 ft [1.7 m] deep    -   Biotrickling zone bed depth: 1 ft [0.3 m]    -   Intermediate Biofilter Zone depth: 2 ft [0.61 m]    -   Biofilter Zone 2 depth: 2 ft [0.61 m]    -   Biotrickling zone Media: HD Q-Pac Manufactured by Lantic        Products Inc. Agoura Hills, Calif. Media volume: 27.5 ft3 [0.78        m3]. Media surface: 5 ft2 [0.46 m2]    -   Intermediate biofliter zone Media: Biosorbens XLD, manufactured        by Biorem Technologies Inc., Guelph, Ontario. Media volume: 55        ft3 [1.56 m3]. Media surface: 10 ft2 [0.93 m2]    -   Biofilter zone Media: Biosorbens XLD, manufactured by Biorem        Technologies Inc., Guelph, Ontario. Media volume: 55 ft3 [1.56        m3]. Media surface: 10 ft2 [0.93 m2]    -   Biotrickling zone trickle rate: 0.7 gpm/ft2 [28.5 L/minute/m2]        of media surface.    -   Biotrickling zone trickle flow rate: 3.5 gpm [13.2 L/minute]    -   Intermediate Biofilter Zone irrigation rate: 0.044 ft3/ft2/d        [0.013 m3/m2/d]. Irrigation average flow rate: 0.44 ft3/d [0.012        m3/d]. Instantaneous flow: 9.2 gpm [34.8 L/minute]    -   Biofilter Zone irrigation Rate: 0.013 ft3/ft2/d [0.004 m3/m2/d].        Irrigation average flow rate: 0.13 ft3/d [0.004 m3/d].        Instantaneous flow rate: 2.8 gpm [10.6 L/minute]    -   Air source: Sewage pump station and sludge storage and        stabilization    -   Air flow rate: 412 cubic feet/minute [11.7 m3/minute]    -   Biotrickling zone empty bed residence time (volume of reactor        without media/air flow rate): 4 seconds    -   Intermediate biofilter zone empty bed residence time: 8 seconds    -   Biofilter zone empty bed residence time: 8 seconds

Procedure:

Contaminated air was passed through the media bed, which had alreadybeen acclimated over an approximately 40 days of operation. Inlet andoutlet samples were taken and analyzed for by St. Croix Sensory usinggas chromatography. Detailed information on the laboratory andmethodology is as follows:

COLUMBIA ANALYTICAL SERVICES, INC. RESULTS OF ANALYSIS Page 1 of 1Client: St Croix Sensory, Incorporated Client Sample ID: TRS OUTLET 1CAS Project ID: P0800552 Client Project ID: BioRem/No. Hatch Is./1849CAS Sample ID: P0800552-003 Test Code: ASTM D 5504-01 Date Collected:Mar. 4, 2008 Instrument ID: Agilent6890A/GC13/SCD Time Collected: NAAnalyst: Zheng Wang/Chris Comett Date Received: Mar. 5, 2008 SamplingMedia: 1.0 L Tedlar Bag Date Analyzed: Mar. 5, 2008 Test Notes: TimeAnalyzed: 12:33 Volume(s) Analyzed: 1.0 ml(s)

Inlet sample results are as follows:

Result MRL Result MRL CAS # Compound μg/m_(—) μg/m_(—) ppbV ppbV7783-06-4  Hydrogen Sulfide 39,000 14 28,000 10 463-58-1  CarbonylSulfide 230 25 94 10 74-93-1 Methyl Mercaptan 1,100 20 540 10 75-08-1Ethyl Mercaptan ND 25 ND 10 75-18-3 Dimethyl Sulfide 78 25 31 10

Outlet sample results are as follows:

Result MRL Result MRL CAS # Compound μg/m_(—) μg/m_(—) ppbV ppbV7783-06-4  Hydrogen Sulfide 14 7.0 10 5.0 463-58-1 Carbonyl Sulfide 22012 88 5.0  74-93-1 Methyl Mercaptan 10 9.8 5.3 5.0  75-08-1 EthylMercaptan ND 13 ND 5.0  75-18-3 Dimethyl Sulfide 35 13 14 5.0  75-15-0Carbon Disulfide 14 7.8 4.5 2.5  75-33-2 Isopropyl Mercaptan ND 16 ND5.0  75-66-1 tert-Butyl Mercaptan ND 18 ND 5.0 107-03-9 n-PropylMercaptan ND 16 ND 5.0 624-89-5 Ethyl Methyl Sulfide ND 16 ND 5.0110-02-1 Thiophene ND 17 ND 5.0 513-44-0 Isobutyl Mercaptan ND 18 ND 5.0352-93-2 Diethyl Sulfide ND 18 ND 5.0 109-79-5 n-Butyl Mercaptan ND 18ND 5.0 624-92-0 Dimethyl Disulfide 25 9.6 6.6 2.5

Removal of total reduced sulphur compounds (TRS), including hydrogensulfide was 99.6%. Removal of organic sulphur compounds (OSC) was 95.5%.

In addition, odour was tested using ASTM E679 & EN13725 methods for St.Croix Sensory. Results are presented below. Very low outlet values of320 and 600 D/T were obtained and average odour reduction exceeded 98%.

The removal rates obtained in a total EBRT of 20 seconds aresurprisingly high when compared with state of art technologies involvinga separate biotrickling filter and biofilter, which may use total EBRTof 45 seconds or higher to achieve similar results. It is believed thatthese results were achieved due to the process configuration and themultistage process disclosed in this invention.

EXAMPLE 5

A horizontal system with a biotrickling zone, an intermediate biofilterzone and a biofilter zone, all arranged in series, with air passing frominlet plenum to the biotrickling zone to the intermediate biofilter zoneand biofilter zone, and finally to an outlet plenum was tested.Dimensions of the horizontal flow system are as follows:

-   -   Location: Preston, Ontario sewage treatment plant    -   Operating Conditions:    -   Overall media Bed Dimensions: 5.5 ft [1.7 m] high×5 ft [1.7 m]        wide by 5 ft [1.7 m] deep    -   Biotrickling filter bed depth: 1 ft [0.3 m]    -   Intermediate Biofilter zone depth: 2 ft [0.61 m]    -   Biofilter Zone depth: 2 ft [0.61 m]    -   Biotrickling Media: HD Q-Pac Manufactured by Lantic Products        Inc. Agoura Hills, Calif. Media volume: 27.5 ft3 [0.78 m3].        Media surface: 5 ft2 [0.46 m2]    -   Intermediate Biofilter zone Media: Biosorbens XLD, manufactured        by Biorem Technologies Inc., Guelph, Ontario. Media volume: 55        ft3 [1.56 m3]. Media surface: 10 ft2 [0.93 m2]    -   Biofilter Zone Media: Biosorbens XLD, manufactured by Biorem        Technologies Inc., Guelph, Ontario. Media volume: 55 ft3 [1.56        m3]. Media surface: 10 ft2 [0.93 m2]    -   Biotrickling filter trickle rate: 0.7 gpm/ft2 [28.5 L/m/m2] of        media surface.    -   Biotrickling filter trickle flow rate: 3.5 gpm [13.2 L/m]    -   Intermediate Biofilter Zone irrigation rate: 0.044 ft3/ft2/d        [0.013 m3/m2/d]. Irrigation average flow rate: 0.44 ft3/d [0.012        m3/d]. Instantaneous flow: 9.2 gpm [34.8 L/m]    -   Biofilter Zone irrigation Rate: 0.013 ft3/ft2/d [0.004 m3/m2/d].        Irrigation average flow rate: 0.13 ft3/d [0.004 m3/d].        Instantaneous flow rate: 2.8 gpm [10.6 L/m]    -   Air source: Sewage head works, primary clarifier, intermediate        pump station    -   Airflow rate: 660 cubic feet/minute [18.7 m3/min]    -   Biotrickling filter empty bed residence time (volume of reactor        without media/air flow rate): 2.5 seconds    -   Intermediate Biofilter Zone empty bed residence time: 5 seconds    -   Biofilter Zone empty bed residence time: 5 seconds

Procedure:

Contaminated air was passed through the media bed, which had alreadybeen acclimated over an approximately 30 days of operation. Inlet andoutlet samples were taken and analyzed using a gas chromatograph withSCD sulphur detector. Results are as follows:

Inlet, ppm by Outlet, ppm by Percent volume volume removal Hydrogen 31.7Non Detect 100% sulphide Methyl mercaptan 0.4 Non Detect 100% Totalreduced 32.1 Non Detect 100% sulphur compounds

The process has succeeded in achieving complete removal of hydrogensulphide and organic sulphur compounds in less than 12.5 s. This is asurprisingly high performance compared with state of art technologywhere this gas stream may require up to 10 second of biotricklingfiltration and 30 second biofiltration to achieve similar performance.It validates the concept of synergistic performance, in particular theuse of horizontal flow, which permits providing variable processconditions along the length of gas flow to achieve complete removal.

EXAMPLE 6

A horizontal gas flow biotrickling filtration system was tested with an8-12 mm expanded glass uncoated media for hydrogen sulphide treatment.The objective was to test performance with a non-compressible mediausing velocities that would typically exist in a horizontal BTF, but ata much higher concentration than was considered acceptable in a verticalbiotrickling filter with this media due to high pressure drops. Thereactor consisted of four equal sections that were independentlymonitored using continuous Odalog® hydrogen sulphide monitors.Dimensions of each reactor section (Sections 1 to 4) were as follows:

Length of bed: 8 inches (0.203 m)

Height of bed: 8 inches (0.203 m)

Width of bed: 8 inches (0.203 m)

Operating conditions were as follows:

Air flow: 9 ft3/minute (15.3 m3/h)

Apparent air velocity: 20.25 ft/min (0.103 m/s)

Recirculating Trickle rate: 1.4 gpm/ft2 (57 L/m2) of bed surface area)

Inlet H2S concentration: 200 ppm v

Temperature: 20-38 C

Recirculating liquid pH: 1.5-2

EBRT were as follows:

Section 1: 2 seconds

Section 2: 2 seconds

Section 3: 2 seconds

Section 4: 2 seconds

The system exhibited improved performance throughout the period withperiodic fluctuations due to operational issues such as exhaustion ofhydrogen supply for short periods, or mechanical breakdown of nutrientfeed system. The following table presents data for two days, as theseare considered typical after the initial acclimation period.

Horizontal BTF performance for two days under stable conditions

Day and Temp Section 1 Section 2 Section 3 Section 4 Time Deg. C.Parameter Inlet Out Out Out Out EBRT, s 2 4 6 8 1 15:40 28 H₂S 199.140.8 8.7 5.7 3.6 2 16:40 25 Concentrations, 205.2 44.9 3.8 0.8 0.1 ppmv1 15:40 28 Removal 80% 96%  97%  98% 2 16:40 25 Efficiency 78% 98% 100%100% 1 15:40 28 Elimination 402.13 241.84 163.77 124.16 2 16:40 25Capacity, g 407.21 255.81 173.08 130.25 H₂S/m³ of media/h 1 16:00 28Pressure Drop, 0.67 0.61 0.67 0.53 2 16:00 25 kPa/m of Bed 0.66 0.470.49 0.56

These results are highly surprising as very high removal efficienciesand elimination capacities were achieved at 2 and 4 seconds EBRT at veryacceptable pressure drop. A stand-alone vertical BTF will typically bedesigned for 12 seconds, and up to 20 seconds to provide 99% removal,and to guarantee this performance under fluctuating load conditions. Ahorizontal BTF, which is integrally connected to a BF, need provide only4 second EBRT even at 200 ppmv inlet concentration to achieve completehydrogen sulphide removal, as any remaining contaminant will be removedin the BF. This will result in a very small BTF section and a smalltotal reactor volume.

While the above description provides examples of one or more processesor apparatuses, it will be appreciated that other processes orapparatuses may be within the scope of the accompanying claims.

1. A process comprising a) passing a gas to be treated through two ormore adjacent zones, the zones differing in one or more of (a) extent ofbiotrickling filter conditions or biofilter conditions, (b) changingmedia characteristics, c) liquid or nutrient recirculation rates orfrequency, (d) pH, (e) gas velocity, (f) retention time along the gasflow passage (g) extent of irrigation if any or (h) cross-sectionalarea.
 2. The process of claim 1 further comprising a step of flowing aliquid vertically through at least one of the zones.
 3. The process ofclaim 2 wherein the liquid comprises water, nutrients, and acid.
 4. Theprocess of claim 2 wherein the liquid is water collected from the bottomof at least one of the zones.
 5. The process of claim 1 furthercomprising a) providing zonal control of process conditions as the gaspasses from an inlet to an outlet and liquid is introduced at the top ofone or more of the zones and flows down by gravity.
 6. A processcomprising, a) providing a gas to a reactor; b) passing the gas in agenerally horizontal direction through a biotrickling zone of thereactor c) passing the gas in a generally horizontal direction through abiofilter zone of the reactor.
 7. The process of claim 6 furthercomprising d) passing the gas in a generally horizontal directionthrough an intermittent biotrickling or biofilter zone positionedbetween the biotrickling zone and the biofilter zone of the reactor. 8.The process of claim 6, further comprising: d) selecting an inert mediafor the biotrickling zone and a biologically active media for thebiofilter zone.
 9. The process of claim 6, further comprising: d)recirculating liquid through the biotrickling zone continuously, and; e)providing once through, and intermittent irrigation of the biofilterzone.
 10. The process of claim 9, further comprising f) passing the gasin a generally horizontal direction through an intermittent biotricklingzone positioned between the biotrickling zone and the biofilter zone ofthe reactor g) providing once-through and intermittent irrigation to theintermittent biotrickling zone, wherein the irrigation provided to theintermittent biotrickling zone is provided at a higher flow or frequencythan the irrigation provided to the biofilter zone.
 11. The process ofclaim 6, further comprising introducing a liquid at a top of thebiotrickling zone, and distributing the liquid evenly across a mediaprovided in the biotrickling zone.
 12. The process of claim 9 furthercomprising introducing irrigation liquid at a top of the intermittentbiotrickling zone or the biofilter zone periodically.
 13. A reactorcomprising a) a gas inlet b) a biotrickling zone in communication withthe inlet; c) a biofilter zone in communication with the biotricklingzone and spaced horizontally from the biotrickling zone; and d) a gasoutlet in communication with the biofilter zone.
 14. The reactor ofclaim 13, further comprising an intermittent biotrickling zone incommunication with and spaced horizontally from the biotrickling zoneand in communication with and spaced horizontally from the biofilterzone.
 15. The reactor of claim 14, wherein the intermittent biotricklingzone is provided between and adjacent the biotrickling zone and thebiofilter zone.
 16. The reactor of claim 13, wherein the reactor isgenerally rectangular.
 17. The reactor of claim 13, further comprisingan inlet gas distribution manifold.
 18. The reactor of claim 17, whereinthe biotrickling zone and biofilter zone are provided adjacent a firstside of the inlet gas distribution manifold, and the reactor furthercomprises: a) A second biotrickling zone provided adjacent a secondopposed side of the inlet gas distribution manifold; and b) A secondbiofilter zone in communication with and spaced horizontally from thesecond biotrickling zone.
 19. The reactor of claim 13, furthercomprising a sump positioned to collect draining liquid from thebiotrickling zone and the biofilter zone, and a recycle pump incommunication with the sump and configured to provide the drainageliquid to the biotrickling zone.
 20. The reactor of claim 13, furthercomprising a an intermediate biofilter zone in communication with andspaced horizontally from the biotrickling zone and in communication withand spaced horizontally from the biofilter zone